1. Technical Field
This disclosure relates to thermosensitive recording materials that are favorably utilized in various fields such as of computer outputs, calculator printers, medical meter recorders, low-speed or high-speed facsimiles, ticket vendors and handy terminals.
2. Description of the Related Art
A number of proposals have been made heretofore for thermosensitive recording materials that have a thermosensitive coloring layer containing at least a leuco dye and a color developer on a support and make use of a coloring reaction between the leuco dye and the color developer. The thermosensitive recording materials have such benefits as (i) complicated procedures like development and fixing are unnecessary, and recording can be carried out by use of relatively simple devices in a short period, (ii) noise is low, and (iii) cost is inexpensive, therefore, have been widely used for copies of books or literatures, and for recording materials of computers, facsimiles, ticket vendors, label printers, recorders, handy terminals, or the like.
The thermosensitive recording materials have been desired for quick color development into high concentration as well as high toughness of colored images and background. In recent years, the thermosensitive recording materials have been frequently used for labels etc. that are importantly required for the reliability of recorded images, in particular, those having high storage stability for plasticizers and fats-oils are demanded that are included into organic polymer materials of packages.
Thermosensitive recording materials have hence been proposed that are provided with a protective layer on the thermosensitive coloring layer. The binder resins in the protective layer are exemplified by polyvinyl alcohol, modified polyvinyl alcohol, or combinations thereof and water-proof agents. For example, a combination of a diacetone-modified polyvinyl alcohol and a hydrazine compound is proposed (see Japanese Patent Application Laid-Open (JP-A) No. 08-151412). However, when the proposed material is employed in the protective layer, there arises such a problem that the coating liquid for the protective layer increases its viscosity with time since a water-proof reaction is promoted under the coated condition.
A thermosensitive recording material is also proposed in which a diacetone-modified polyvinyl alcohol is employed as a binder resin of the protective layer and a hydrazine compound is included into the thermosensitive coloring layer (see JP-A No. 11-314457). However, this proposal suffers from insufficient water resistance of the protective layer, viscosity rise of the coating liquid for thermosensitive coloring layer, and/or coloring inhibition at the thermosensitive coloring layer due to the hydrazine compound.
A thermosensitive recording material is also proposed in which a polyvinyl alcohol copolymer containing diacetone acrylamide as its monomer, a water-soluble hydrazine compound, and a water-soluble amine are used (see JP-A No. 10-87936). However, when the proposed material is employed in the protective layer, there arise such problems as the background develops a color due to an adverse effect of the water-soluble amine on the thermosensitive coloring layer, pH control is difficult, and viscosity rise of the liquid may be promoted depending on the amount of the water-soluble amine.
In order to improve these problems, JP-A No. 2002-283717 discloses a thermosensitive recording material in which a polyvinyl alcohol having a reactive carbonyl group, a hydrazide compound, and a basic filler are combined. This proposal may improve the problem of viscosity rise.
However, the thermosensitive recording material, which employs the polyvinyl alcohol having a reactive carbonyl group and the hydrazide compound as described in JP-A No. 2002-283717, suffers from such a problem that images printed with aqueous flexographic ink are easily removed away by an external force upon contacting the images with water for a long period; and this problem is currently demanded to be solved.